Differential ion mobility spectrometry (DMS) is a technique based on the principles of ion mobility spectrometry (IMS). In IMS, ions are guided by an axial uniform electric field through a gas medium at constant pressure. The combined action of the driving force of the electric field accelerating the charged species and the damping force introduced by collisions between ions and the gas molecules results in an average drift velocity of the ions in the direction of the applied electric field.
Ion mobility is defined as the ratio of the average drift velocity of an ion group injected into the IMS cell over the applied electric field, K=uAV/E. Therefore, the drift time of an ion through a given length is determined by the applied electric field and the mobility; the latter reflects the ion's collision cross section as well as the nature of the interactions between ions and the molecules comprising the gas medium. Ions having different collision cross sections, and depending on the nature of the interaction with the gas medium, will resolve into groups drifting with different average velocities through the cell. Separation in IMS relies primarily on variations in the volume/charge ratio of the ions.
Recent developments in IMS have been mainly driven by applications involving the structural elucidation of macromolecules in conjunction with the determination of their molecular masses using mass spectrometry (MS). Additional features that establish IMS as an indispensible tool in the analysis of complex samples is the separation of isobaric forms of compounds (that is, compounds having the same m/z ratio) and also the enhancement of the signal-to-noise ratio observed in a mass spectrum.
In so-called hyphenated IMS-MS instruments, the ion mobility drift cell is attached to the front-end of the mass spectrometer, externally to the mass spectrometer's vacuum enclosure, and operated at ambient pressure. Consequently, mobility separation is limited to ions generated in atmospheric pressure ionization sources. Atmospheric pressure IMS suffers from low efficiency of transmission of ions into the mass spectrometer's vacuum enclosure because diffusion causes expansion of the ion beam, which adversely affects sampling efficiency at the MS interface as ions must pass through a small aperture, typically 0.2 to 0.5 mm diameter.
Despite the fact that diffusion becomes more dominant at lower pressures and ion losses can become significant, ion optical devices can be inserted after the IMS device to re-gather ions. This has permitted the development of low pressure and vacuum IMS, which has considerably extended the range of IMS instruments and techniques available for the analysis of complex mixtures. Intermediate pressure IMS cells are compatible with virtually any vacuum ion source, in addition to available atmospheric pressure ionization sources. Separation of ions based on ion mobility has been performed at pressures as low as 0.1 mtorr. As with ambient pressure IMS, the ions exiting the IMS drift cell and ion optics can be delivered to the front-end of a mass spectrometer.
Ion mobility, K, varies non-linearly with variations on the applied electric field and pressure. This dependence is usually approximated by a series expansion of the mobility K in even powers of the parameter E/N where E is the electric field and N is the number gas density per Eq. (I) [E. A Mason, E. W. McDaniel, Transport Properties of Ions in Gases; Wiley, 1988]:K(E/N)=K(0)[1+α2(E/N)2+α4(E/N)4+ . . . ]  (I)
The ion mobility at the zero field limit, K(0), is used to define the threshold below which the value of the average drift velocity scales linearly with electric field, that is, the ion mobility K(0) is constant and velocity is directly proportional to electric field, uAV=K(0) E. Drift cells operated at atmospheric pressure are usually operated below the zero field limit and the electric field gradient required to guide ions through the gas is greater as compared to drift cells operated at reduced pressures where the value of E/N may extend into the non-linear range of K. Ions are categorized using Eq. (I) and the corresponding mobility coefficients, or alpha coefficients, which determine the dependence of K on E/N. For A-type ions α2>0, α4>0 and mobility increases with E/N. The effect is reversed for C-type ions where the mobility decreases with E/N and α2<0, α4<0. A more complex behavior is obtained for B-type ions where α2>0, α4<0. The Townsend unit, Td, has been introduced to depict that the fundamental character of ion-molecule interactions in ion mobility is revealed by the dependence of K on the ratio of parameters E/N, where, 1 Td=10−21 Vm2.
Several techniques for separating ions based on the mobility properties of ionic species have been developed since the early work performed on drift cell IMS. In particular, differential mobility spectrometry (DMS) [I. A. Buryakov, et al, Int. J. Mass Spectrom. Ion Processes 1993, 128, 143], also known as field asymmetric ion mobility spectrometry (FAIMS) [R. W. Purves, et al, Rev. Sci. Instrum. 1998, 69, 4094], relies on the dependency of the ion mobility, K, on the applied electric field and number gas density, E/N. In contrast to IMS, the ions in DMS are entrained in a gas stream and oscillate in the presence of a periodic asymmetric waveform that alternates between a high-field and a reversed low-field. The electric field is applied perpendicularly to the direction of gas flow. Ions experience an average net displacement per waveform cycle depending on the differences between high- and low-mobility. This results in the ions drifting progressively off-axis and discharging on electrodes confining gas flow. The displacement can be compensated by a DC voltage and ions of a given mobility dependence can be transmitted successfully through the device. A spectrum is generated by scanning the compensation voltage at fixed amplitude and waveform frequency and collecting the transmitted ions either by using an electrometer or introducing them into the front end of a mass spectrometer.
Two principal DMS systems have been developed, depending on their ability to focus ions in the direction transverse to the gas flow. In the first type, ions are carried by gas flow confined between two concentric cylinders of different radii in a coaxial arrangement. The asymmetric waveform and the compensation voltage are usually applied to the inner electrode. The logarithmic field established between the two cylindrical electrodes has the ability to focus ions transversally and maintain high transmission at increased waveform amplitudes [R. Guevremont, R. W. Purves, Rev. Sci. Instrum. 1999, 70, 1370]. In the second configuration ions are forced to oscillate between two parallel plates, one of which carries the asymmetric periodic waveform and the compensation voltage while the opposite electrode is maintained at ground potential. The dipole field formed between the plates has no focusing properties and the number of ions lost on the electrodes is approximately proportional to the amplitude of the asymmetric waveform. Transmission through such a dipole field is possible for all types of ions, the types being categorized depending on the type of the non-linear dependence of K on E/N, in contrast to the cylindrical design where transportation becomes selective, that is, ions of a certain type can only be transmitted for a given waveform.
The present inventors have found that the performance and applications of DMS so far is limited due to a number of disadvantages associated with this relatively new technology. In particular, unlike IMS, the DMS devices described in the literature have been exclusively operated at ambient or sub-ambient pressures and interfaced externally to the vacuum enclosure of a mass spectrometer.
Generally, the pumping rate provided by the inlet of the MS (e.g. a capillary or critical orifice) is in the region of 1 L min−1, which has been found to be a convenient rate for pumping air slowly through the gap between the plates of a DMS or a FAIMS device. This provides the necessary laminar flow conditions for separation to occur.
Nevertheless, a disadvantage of operating at a fixed flow rate is that the predetermined residence time of the ions through the DMS cannot be easily adjusted for enhancing instrument performance. This is particularly true in the case where the separation gap between the DMS electrodes is also fixed. Operating the DMS at ambient or near ambient pressure and establishing high-field conditions (˜100 Td) sufficient for inducing separation requires the minimum possible separation distance between the electrodes, which in turn limits the sampling efficiency of the system and compromises sensitivity. In particular, sampling by a MS of electrosprayed ions through the narrow gap of a DMS device becomes problematic. Furthermore, it is demonstrated experimentally that the transmitted ion current cannot exceed ˜10 pA, which is significantly lower than the ion current generated in an electrospray ionization source [Shvartsburg at al, J. Am. Soc Mass Spectrom. 2005, 16, 2-12]. In summary, the number of ions available for analysis in the MS is much lower because of ion losses and restrictions on ion flow caused by the DMS.
To date, DMS devices are coupled to atmospheric pressure “soft” ionization sources, and in particular to the electrospray ionization source operated at relatively low flow rates ˜1 μL min−1. This limitation is mainly imposed by the formation of bigger droplets when spraying at the higher flow rates, which, unless sufficient evaporation is allowed to occur, can significantly degrade the performance of the DMS. Since operation of such devices at ambient conditions are incapable of attaining the desired performance, accommodation of such high flow rates required for high throughput LC MS analysis using DMS as the front-end in MS platforms remains a goal.
Furthermore, the operation of a DMS device at ambient pressure is restricted to clean samples and liquid chromatography (LC) buffers not containing involatile salts. The direct analysis of “dirty” samples such as biological fluids can quickly compromise the DMS performance. Robust ionization sources have been developed to tolerate these types of samples, together with the involatile buffers used in aiding LC separations, however, they remain incompatible with the DMS interface to the MS.
Another limitation of the current DMS technology is the poor resolution, measured by the peak width in terms of the compensation voltage, which is limited to ˜20 and appears to be significantly lower than that obtained in drift cell IMS. Methods to improve resolution are compromised by the narrow range of E/N at which DMS has been operated to date.
In a FAIMS device described in US 2003/0020012, parent ions generated from a sample undergo mass analysis in the normal way and then fragment ions produced by a collision cell are subjected to FAIMS separation. This requires the pressure in the FAIMS device to be compatible with the collision cell operating pressure. Specifically, parent ions are selectively transmitted through a first mass analyzer in a low pressure chamber, injected into a collision cell operating in a second pressure chamber operating at increased pressure (which second pressure chamber is located within the low pressure chamber) wherein fragmentation of the parent ions occurs in a collision cell.
Subsequently, the fragment ions are filtered by a FAIMS device prior to injection of the ions from the second pressure chamber back into the low pressure chamber for the second stage of mass analysis.
This geometry is intended only to separate fragment ions with equal ratios of m/z (isobaric ions) which would otherwise appear as a single spectral line when measured in the second mass analyzer. The pressure range established in the FAIMS device is therefore limited by the operational pressure of the collision cell. Indeed, the dedicated collision gas supply provided to the second pressure chamber dictates the pressure of the FAIMS device. Accordingly, a range of operating pressures is not available and hence the range of accessible E/N ratios is narrow.
In another DMS arrangement described by E. G. Nazarov at al, Anal in Chem, 2006, 78, 7697, ions are transported through a planar electrode system where pressure within the DMS may be adjusted by means of a system of flow controllers, needle valves and a miniature pump. The DMS is situated externally to a mass spectrometer and ions transported successfully through the gap between the planar electrodes are deflected by a DC bias into a 2 mm inlet hole and toward the inlet orifice of the mass spectrometer. Using this system the effect of pressure was investigated in the range of 0.4-1.55 atm (405-1570.5 mbar). A pressure of 0.6-0.8 atm was found to provide reduced dimerisation and high resolution. The present inventors have observed that transportation of the ions from the DMS to the mass spec relies to a great extent on gas flow and reducing the pressure differential across the MS interface has a significant effect on sensitivity. Thus, lowering the pressure below that studied by Nazarov et al would have an adverse affect on transport efficiency of the ions from the DMS through the inlet capillary or orifice of the MS.
Thus, at present, DMS and FAIMS devices are operated at and/or near ambient pressure and the value of E/N is limited to ˜100 Td (1 Td=10−21 Vm2), which corresponds to ˜1220 V across a 0.5 mm gap at ˜1 atm=1013.25 mbar and 300 K. At these pressures breakdown events impose an upper limit to the amplitude of the waveform and therefore restrict the accessible range of the ratio E/N. Furthermore, ion transport from the DMS to the MS is inefficient.